问题 问答题 论述题

为什么说信号电流频率的选择是轨道电路防干扰的一个重要问题?

答案

参考答案:

在频率的选择上,应该首先说明的是:即使在直流电力牵引时信号电流频率选用50Hz也是不相宜的。由于各种电力传输线路的设置,车站照明广泛地采用交流电,牵引变电所整流器工作的不正常等,所有这些因素都迫使我们担心频率5Ohz交流电的影响而必然要求我们选用其他的信号频率。

因此,最好的方案是选用能适合于任何电力牵引制度的信号电流频率。在频率选择上,还应该说明的是:一般情况下只要信号频率选成区别于50Hz牵引电流的基波和谐波(只考虑奇次谐波),使之干扰很小,不能错误地动作接收设备即可,但在实际上,当牵引电流的波形上不对称时,将出现偶次谐波;另外,电网的工频50Hz有正或负的0~2Hz的漂移,这就给采用较高的频率制式带来困难,因有频漂,工频的奇次谐波与额定频率的偶次谐波相差值将随偶次谐波次数的增大而缩小。

例如48Hz和52Hz的15次谐波分别为720Hz和78OHz,它与7O0Hz比和8OOHz的偶次谐波只相差2OHz,或仅相差2.9%~2.5%,高于15次的奇次谐波与相应额定频率的偶次谐波更加接近而不易区分,尽管谐波的幅度随谐波次数的增加而急剧地减小,但对于接收灵敏度高而牵引电流的干扰很大时,即使不考虑偶次谐彼的干扰,由于频漂的奇、偶相近,也给选定信号频率带来一定的困难。

由此可知,选定的信号电流频率需要既不同于电力牵引电流的基波,也不同于奇次和偶次谐波,且在接收设备的设计和制造时使之具有鲜明的频率选择性,以确保轨道电路可靠地正常动作,是比较理想的。

因此,电气化区段的轨道电路。除了应满足一般轨道电路的基本要求外,尚有一项重要的技术要求,即要在受到牵引电流(基波及谐波)的干扰下,仍能保证完成轨道电路功能的防护干扰的能力

阅读理解
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     The following are four kinds of medicine at home. We should know and remember how to use them.
I. Take the medicine with water. One or two pills (片) every eight hours. Don't take more than six pills in
    24 hours. For children under six, go to ask the doctor.
II. Three times a day and one pill a time. One pill before breakfast, one before 11: 00 and one before
    sleep. Not for children under six and persons with stomachache.
III. The medicine is for a person with a toothache. Two pills a time a day before sleep for adults (成人).
    Half (一半) a pill for children under 12.
IV. Take the medicine four times a day. Four pills a time for adults with a cold. Two pills a time for
    children under 10. Take the medicine before each meal and before sleep.
1. Which medicine is for people with a toothache?
A. I         
B. II      
C. III

2. People with a _____ should not take medicine II.
A. toothache  
B. cold        
C. stomachache

3. Medicine I should not be taken more than 6 pills in _____.
A. 6 hours          
B. 8 hours      
C. 24 hours      

4. If a girl is 7 years old, she should take _____ pills of medicine IV a day.
A. 5                
B. 8        
C. 10

5. Which of the following is TRUE?
A. None of them are good for children under 6.
B. Three kinds of the medicine should be taken 3 times a day.
C. Three kinds of the medicine should be taken before sleep.      
填空题